Composite Metal

ABSTRACT

A metal composite comprising a milled and compacted mixture of powdered aluminium or aluminium alloy and ceramic particles, wherein, on loading of the aluminium with the ceramic particles, the ceramic particles are of an average size of between 0.85 μm and 0.6 μm.

The present invention relates to a composite metal.

It is known that metal alloys exhibit differing properties in accordancewith the different constitution of the alloys. Further it is known thatnon-metallic constituents can have significant effects. Small amounts ofcarbon changes soft iron into strong and tough steel, although webelieve that the mechanism in the case of steel is different from thatof the present invention.

We have combined much larger quantities and sizes of ceramic particles,compared with carbon inclusions in steel, into aluminium alloys andachieved significant increases in strength, without compromise toductility and machinability.

Our method of forming our composite aluminium alloys is essentially asdescribed and claimed in U.S. Pat. No 4,749,545, namely preparing metalmatrix composites comprising a hard material selected from siliconcarbides, silicon nitrides, silicon oxides, boron carbides, boronnitrides and boron oxides, and a lightweight component selected fromaluminium, magnesium and alloys of either, the method comprising:

-   -   intimately mixing using a high energy milling technique a powder        of the hard material and either aluminium or magnesium in its        powder form to produce a uniform powder mixture and    -   compacting the powder mixture at elevated temperatures.

Broadly we load aluminium alloy with 3 μm, i.e. 3micron or 3 micrometre,particles of silicon carbide, normally at 25% or 40% by volume. Weachieve good strength to weight ratios whilst retaining machinabilityand ductility, which enables forging of parts from our compositealuminium alloys.

As recorded in U.S. Pat. No. 6,398,843, we have also proposed the use ofceramic particles an order of magnitude smaller, that is of up to 0.4μm. However we experience difficulty with loadings higher than 10% inthat it is difficult to distribute the ceramic particles evenlythroughout the aluminium. Further, the particles tend to agglomerate,resulting in weak spots in the finished composite metal.

We have now unexpectedly discovered that we can load aluminium alloyswith particles not much larger than 0.4 μm to at least some of the sameloadings that we use with 3 μm particles.

The object of the present invention is to provide metal composite.

According to the invention there is provided a metal compositecomprising a milled and compacted mixture of powdered aluminium oraluminium alloy and ceramic particles, wherein the ceramic particles areof an average size of between 1.0 μm and 0.5 μm.

Preferably the particles will be between 0.85 μm and 0.6 μm and inparticularly between 0.75 μm and 0.65 μm. The single most preferredparticle size is 0.7 μm.

We anticipate that the invention will be operable with pure aluminiumand with aluminium alloys having single or joint alloy additions of Cu,Mg, Mn, Li, Zn, Si, Zr, Cr, Fe, Ni, Ti. We prefer to use medium strengthalloys, in particular aluminium alloys including Cu, Mg, Mn. Whereenhanced corrosion resistance (achieved by limiting Cu content) and/orenhanced ductility is required, as with relatively high ceramic particleloading, we prefer to use low strength alloys, in particular aluminiumalloys including Mg, Si some copper.

In particular we prefer AA2124 as such a medium strength matrix alloyand AA6061 as such a low strength matrix alloy. These have thecompositions shown in the following Table 1:

TABLE 1 Preferred Matrix Alloy Compositions (Weight %) AA2124 AA6061 Cu3.6-4.9 0.15-0.40 Mg 1.2-1.8 0.8-1.2 Si 0.20 max 0.4-0.8 Fe 0.30 max0.70 max Zn 0.25 max — Mn 0.4-0.9 — Cr 0.1 0.04-0.35

Further we anticipate that the invention will be operable with siliconcarbide, boron carbide, aluminium oxide and other ceramics based onmetal carbides, oxides or nitrides. Silicon carbide is our preferredceramic on economic grounds. Again, we anticipate that the inventionwill be operable for volume percentage loading of ceramic in thealuminium or aluminium alloy of between 15% and 50% and preferably 18%and 40%. The most preferred volume percentages are 18%, 25% and 40%.

Where we specify a ceramic by diameter, we expect that in accordancewith industry standards, the size distribution will be Gaussian withupper and lower quartiles at 125% and 75% of the quoted particle size.

To help understanding of the invention, a specific embodiment thereofwill now be described by way of example and with reference to theaccompanying drawings, in which:

FIG. 1 is a chart showing our existing and proposed loadings and theloading that would be expected if the same correlation were to beapplied to use of ceramic particles sized in accordance with theinvention;

FIG. 2 are stress-strain plots for the matrix alloy without anyreinforcement, our conventional 3.0 μm ceramic particle loaded aluminiumalloy composite and 0.7 μm ceramic particle loaded metal composite;

FIG. 3 is a plot of elongation to failure against ceramic particleloading in volume %.

Referring to referring to FIG. 1, our current aluminium alloy compositesuse from 18% to 40% by volume of 3 μm particles. This is shown at I inFIG. 1. We have also proposed use of up to 0.4 μm particles and havebeen able to produce composites with good ceramic distribution andwithout agglomerations, but in practice we have to limit the ceramiccontent to no more than 10% silicon-carbide by volume. This is shown atII in FIG. 1. Our experience suggests that loading with more than 10% of0.4 μm particles leads to lack of homogeneity and performancedegradation, in particular to occurrence of unpredictable weak spots inthe alloy.

Our new aluminium alloy composite preferably uses 0.7 μm particles.Assuming a straight line correlation, we would have expected to havebeen able to use only up to 13.5% of such particles, as shown at III. Infact we have been able to use 40%, shown at IV, which is unexpected.Indeed it appears that we can use the same loadings by volume as withparticles, despite the particle size being much closer to 0.4 μm with itloading limitation than 3 μm.

Referring to FIG. 2, the stress/strain curve are shown for:

-   -   1. Our preferred matrix alloy AA2124 alone,    -   2. The same matrix alloy loaded conventionally with 25% of 3.0        μm ceramic particles,    -   3. The same matrix alloy with the improved loading of 25% of 0.7        μm ceramic particles.

Not only is the improved composite metal much stronger than the matrixalloy alone, but also it has 30% improvement in yield strength Y, orelastic limit, just past the limit of proportionality, compared with ourconventional composite metal for the same loading of larger particles.

In the greater portion of the elastic region, the two composite metalsbehave similarly, but in the plastic region, whilst the shapes of thecurves are similar, the 0.7 μm ceramic particles deforms at higherstress corresponding to the 30% improvement in yield strength.

The density, Young's modulus, strain to fail, yield strength andultimate strengths of plain alloy and the 3.0 μm and 0.7 μm, at 18 and25 volume % loaded composite aluminium alloys are as follows in Table 2for as-compacted billet:

TABLE 2 Comparison of Properties of Existing and Improved MetalComposites - as compacted billet. Baseline 3.0 μm 0.7 μm 3.0 μm 0.7 μmAlloy SiC—18% vol % SiC—18% vol % SiC—25% vol % SiC—25% vol % PropertyComparison 2124 2124/SiC/18p 2124/SiC/18p 2124/SiC/25p 2124/SiC/25p (T4)as-compacted as-compacted as-compacted as-compacted billet (T4) billet(T4) billet (T4) billet (T4) Density (g/cc) 2.77 2.85 2.85 2.88 2.88Tensile Modulus 72 100 100 115 115 (GPa) Strain to Failure 12 4 4 2 2(%) 0.2% Yield Stress 325 420 490 460 600 (MPa) Ultimate Tensile 470 555600 580 680 Strength (MPa)

From Table 2, it will be noted that the properties of the 0.7 μmparticle composites alloys are for each of the properties not onlygreatly improved over the plain alloy (with the exception of theexpected decrease in strain to failure) but also either the same as orimproved with respect to the properties of the 3.0 μm particlecomposites alloys.

Again the density, Young's modulus, strain to fail, yield strength andultimate strengths of of plain alloy and the 3.0 μm and 0.7 μm, at 18%loaded composite aluminium alloys are as follows in Table 3 for extrudedbar:

TABLE 3 Comparison of Properties of Existing and Improved MetalComposites - as extruded. Baseline 3.0 μm 0.7 μm Alloy SiC—18% vol %SiC—18% vol % Property Comparison 2124 2124/SiC/18p 2124/SiC/18p (T4)Extrusion (T4) Extrusion (T4) Density (g/cc) 2.77 2.85 2.85 TensileModulus 72 100 100 (GPa) Strain to Failure 12 7 7 (%) 0.2% YieldStrength 325 420 480 (MPa) Ultimate Tensile 470 620 680 Strength (Mpa)

Again Table 3 shows the same or improved properties through use of 0.7μm particle size.

Looked at differently, from Table 2 and Table 3 it can be seen thatthere is a significant advantage to strength using in the 0.7 μm ceramicreinforcement and that this is achieved without the detriment to thestrain to failure, as might be expected.

Turning to FIG. 3, the strain to failure or elongation is shown as afunction of loading of the matrix metal with different sizes of ceramicparticles. The plots 4,5 for 3.0 μm and 0.7 μm are very similar anddistinct from the 0.4 μm particle size plot 6. It is surprising in viewof the distinction between the 3.0 μm and 0.4 μm plots that the 0.7 μmplot tracks 3.0 μm plot so closely.

It is our experience that our composite aluminium alloys having themechanical properties shown in FIG. 2 are readily machinable, but toolwear may be evident and increase the cost of manufacture. Therefore weexpect the composites of the invention to be more readily machinablewith less tool wear offering a significant economic advantage.

In summary of the properties of the specific aluminium alloy compositesdescribed above, use of 0.7 μm silicon carbide particle reinforcementdirectly in place of 3.0 μm reinforcement in AA2124 achieves enhancedresults, which is surprising in view of your earlier experience with 0.4μm silicon carbide particle reinforcement.

1. A metal composite comprising a milled and compacted mixture ofpowdered aluminium or aluminium alloy and ceramic particles, wherein, onloading of the aluminium with the ceramic particles, the ceramicparticles are of an average size of between 0.85 μm and 0.6 μm.
 2. Ametal composite as claimed in claim 1, wherein the ceramic particles areof an average size of between 0.75 μm and 0.65 μm.
 3. A metal compositeas claimed in claim 1, wherein the ceramic particles are of 0.7 μm insize.
 4. A metal composite as claimed claim 1, wherein the aluminium ispure aluminium.
 5. A metal composite as claimed in claim 1, wherein thealuminium alloy is one having single or joint alloy additions of Cu, Mg,Mn, Li, Zn, Si, Zr, Cr, Fe, Ni, Ti.
 6. A metal composite as claimed inclaim 6, wherein the aluminium alloy is a medium strength alloyincluding Cu, Mg and Mn.
 7. A metal composite as claimed in claim 7,wherein the medium strength alloy is AA2124,
 8. A metal composite asclaimed in claim 6, wherein the aluminium alloy is a low strength alloyincluding Mg, Si and Cu.
 9. A metal composite as claimed in claim 6,wherein the low strength alloy is AA6061.
 10. A metal composite asclaimed in claim 1, wherein the ceramic particles of silicon carbide,boron carbide or aluminium oxide.
 11. A metal composite as claimed inclaim 1, wherein the volume percentage loading of ceramic particles inthe aluminium or aluminium alloy is between 15% and 50%.
 12. A metalcomposite as claimed in claim 1, wherein the volume percentage loadingof ceramic particles in the aluminium or aluminium alloy is between 18%and 40%.
 13. A metal composite as claimed in claim 1, wherein the volumepercentage loading of ceramic particles in the aluminium or aluminiumalloy is 18%, 25% or 40%.
 14. (canceled)